The use of ion exchange resins, usually in the form of spherical beads, for selective removal of dissolved mineral constituents from water is generally well known. These dissolved constituents may either be positively charged ions (so-called cations) or negatively charged ions (so-called anions).
The presence of certain ions in domestic and industrial waters is known to be particularly undesirable. Thus the cations of calcium (Ca.sup.2+) and magnesium (Mg.sup.2+) contribute to so-called "hardness" of water, while dissolved anions such as sulphate (SO.sub.4.sup.2-) can contribute to corrosion and scaling problems in industrial applications. The occurrence of the abovementioned ions is common in waters associated with mining operations, making disposal of such water problematic.
Ion exchange resins generally selectively adsorb cations or anions onto the surface of the resin beads and are accordingly catergorised as cationic or anionic resins. During use the resins become progressively loaded with ions being removed from the water passing the resin beads. Periodic regeneration accordingly becomes necessary in order to strip these ions from the resin in order to make it fit for use again.
Resin regeneration generally involves taking the resin out of service and bringing it into contact with an aqueous liquor containing at least one reagent capable of removing the adsorbed ions from the resin. Sulphuric acid (H.sub.2 SO.sub.4) is a known suitable reagent for regenerating cationic resins, while line (Ca(OH).sub.2) is known for its use in regenerating anionic resins. The advantage of these reagents in relation to other known reagents lies in their comparatively low cost, which makes them suitable for use in large-scale water treatment installations.
Regeneration is generally effected by bringing a resin into contact with a regenerating liquor. This may be an aqueous solution of sulphuric acid or lime, depending on the type of resin being treated. In the case of a regenerating liquor comprising lime, this may only be partially dissolved, the balance being dispersed in water as finely as dispersed particles. Whereas sulphuric acid is used in regenerating liquors in a fully dissolved state, regenerating liquors comprising lime generally contain a fraction of undissolved lime particles which remain dispersed within the liquor throughout the regeneration process.
The use of the abovementioned reagents becomes problematic, however, whenever any of the ions adsorbed onto the resin insoluble products in conjunction with the regenerating reagent. The regeneration of a cationic resin loaded with calcium ions, by means of a regenerating liquor comprising sulphuric acid is a typical example. This can be illustrated by means of the following mechanism EQU R-Ca+H.sub.2 SO.sub.4 (aq).fwdarw.R-H.sub.2 +CaSO.sub.4,
where
R-Ca represents the cationic resin loaded with calcium ions; PA1 H.sub.2 SO.sub.4 (aq) represents an aqueous solution of sulphuric acid, as used in the regenerating liquor; PA1 R-H.sub.2 represents the regenerated resin; and PA1 CaSO.sub.4 represents gypsum, which is poorly soluble in water. PA1 R'-SO.sub.4 represents the anionic resin loaded with sulphate ions; PA1 Ca(OH).sub.2 (aq) represents an aqueous solution of lime, as used in a regenerating liquor; PA1 R-(OH).sub.2 represents the regenerated resin; and CaSO.sub.4 again represents gypsum. PA1 introducing a liquor for treating the resin in substantially vertical upflow into a treatment zone in order to produce a fluidised bed comprising the resin and insoluble particulate matter interspersed with the each other; and PA1 separating the resin from the particulate matter through entrainment of the latter by the liquor from being withdrawn from the fluidised bed in a substantially horizontal flow direction. PA1 a vessel defining a resin treatment zone into which the resin is receivable; PA1 a liquor inlet arranged below the resin treatment zone for introducing liquor for treating the resin in substantially vertical upflow in order to allow a fluidised bed of resin to be produced in the treatment zone; and PA1 separating means extending into the treatment zone whereby, in use, insoluble particulate matter interspersed with the resin is allowed to be entrained from the treatment zone by the liquor flowing in a substantially horizontal flow direction while the resin is retained in this zone.
The gypsum tends to precipitate from solution in the form of minute hydrated mineral particles, generally described by the chemical formula CaSO.sub.4.xH.sub.2 O. These particles precipitate on the surface of the resin being regenerated, rendering the resin at least partially ineffective for further cation removal.
Anionic resin which is regenerated with an aqueous lime solution can suffer similar deterioration when it is loaded with anions such as sulphate (SO.sub.4.sup.2). This is illustrated by the following mechanism: EQU R'SO.sub.4 +Ca(OH).sub.2 (aq).fwdarw.R'-(OH).sub.2 +CaSO.sub.4,
where
The precipitation of gypsum and other insoluble products formed by similar mechanisms tends to lead to long-term resin deterioration by successive regeneration steps. This problem can be counteracted by the provision of small particles of insoluble regeneration product (so-called seeding particles) interspersed with the resin at the commencement of each regeneration step. It has been found that as further regeneration products such as gypsum are formed, these tend to precipitate preferentially on the seeding particles, leaving the resin substantially uncontaminated at the end of each regeneration step.
It is an object of the present invention to provide a process and apparatus which are particularly suited for the treatment of ion exchange resin having insoluble particulate matter such as the seeding particles, or undissolved lime particles, interspersed with it.